Apparently the rule is as follows: When transiting (anywhere in the world) you will be allowed to take your liquid duty free purchases in your hand-luggage provided it is in the original sealed bag with the invoice sealed in as well and it is within 24 hours of the time of purchase.

At least that is what I was told two, three weeks ago in FRA. They definitely accepted it as such in JNB. My purchases were in the sealed bag and no questions were asked.